Are there walls on both sides of the 8 stud space?
For example you could use the those walls to start your arch and then that would give you abour 3 stud wide openings to allow a minifigure to walk through. Basically you'd make a "Y" shape in the middle with a "r" like shape coming of the those walls towards the "Y" in the centre.
Red = Walls with studs facing in at top of arch and up.
Blue = 1x2 tile with 1 stud
Green = 1x1 column (or pole just rotate orange brick or insert half pin into bottom.)
Purple = 1x1 Cheese slopes
Grey = 10 stud long plate, tile, or brick
Orange = 1x1x1 multi stud brick